1. Field of the Invention
This invention relates to filtration apparatus of the type which has a plurality of adsorber beds used to adsorptively filter contaminants from a fluid stream.
2. Description of the Prior Art
Filtration systems have been employed to remove radioactively-contaminated materials, such as radioactive iodine, from off-gases emanating from nuclear installations. Such filtration systems typically employ a plurality of adsorber beds which are filled with a solid, particulate adsorbent, such as granulated charcoal. In such systems, withdrawal of spent adsorbent has presented a serious problem, either because of the hazards presented with some prior withdrawal systems or because of the inefficient use, and therefore increased expense, of other systems which require the accumulation of inordinately large volumes of spent adsorbent which is not useful in the filtration process.
Because of the unique problems presented in handling radioactively contaminated adsorbent, pneumatic discharge apparatus typically employed in other arts has not been successfully used to empty spent adsorbent from filtration beds for nuclear off-gases. The two systems shown in U.S. Pat. Nos. 3,583,768 and Re 27,471 for emptying railway hopper cars are typical.
It is, of course, theoretically possible to use one point of withdrawal at the bottom of a filter apparatus having a plurality of adsorber beds. This could be done by using a cone-shaped withdrawal hopper located immediately below the adsorber beds. Such a system, however, would be prohibitively large and uneconomical because it would require an inordinately large volume of charcoal to accumulate at a location where it was not useful. Additionally, there would be a significant increase in the initial capital cost necessitated by the fabrication of such a large hopper.
The use of multiple cone- or trough-shaped withdrawal hoppers has also been suggested, but this also results in an inordinate accumulation of adsorbent and can also increase the possibility of accidental spillage or hazard to the operator.
Alternatively, filters can be fabricated with sloping walls so that spent charcoal naturally spills over into a collection trough, such as that described and shown in U.S. Pat. No. 3,795,090. This can limit the design freedom, however, and can also mean that fabrication will be more complex and more costly if the walls are limited to such a sloped configuration.
A withdrawal device which has been widely used is an externally inserted vacuum tube. This presents a severe hazard, however, because the vacuum tube can become radioactively contaminated after minimum usage. Accidental spillage of radioactive adsorbent is also possible as the probe is inserted and removed. Additionally, when the service space is limited, the use of a vacuum tube long enough to withdraw spent adsorbent from the entire length of a filter is often impossible. In such instances, two or more units have to be joined, which inordinately increases the probability of a problem in withdrawal of radioactive adsorbent.
Two more recently developed systems for withdrawing spent adsorbent from filtration systems without the necessity of inserting a probe have been described in the patent literature. In U.S. Pat. No. 3,881,899, a withdrawal apparatus having a projecting downspout into which spent adsorbent can fall is used in conjunction with a pneumatic conveying system. U.S. Pat. No. 3,925,046 discloses a pneumatic conveying system also, but in this case, the spent adsorbent is sucked up into an inverted cup member. Both of these systems have the disadvantages of requiring multiple outlets and excessive distance below the adsorber beds for large adsorber units, resulting in more costly construction and larger volumes of unused adsorbent.